This invention relates to a process for the production of olefins, especially a two stage process wherein, in the first stage, heavy petroleum fractions are hydrogenated in the presence of hydrogen and a hydrogenation catalyst and, in the second stage, the thus-hydrogenated fractions are subjected to thermal cracking in the presence of steam.
Such a process has been known from DOS [German Unexamined Laid-Open Application] 2,164,951, an also U.S. Pat. Nos. 3,720,729 of Sze et al, Mar. 13, 1973 and 3,781,195 of Davis et al, Dec. 25, 1973. In contrast to a direct thermal cracking of heavy crude oil fractions, wherein a great amount of pyrolysis oil, tar, and coke is produced in addition to a minor yield, it is possible by means of the conventional method to produce, by catalytic hydrogenation of the crude oil fractions prior to thermal cracking, hydrogenated fractions which are suitable for thermal cracking.
In the process of DOS No. 2,164,951, the hydrogenation catalysts employed were combinations of nickel or cobalt with molybdenum or tungsten on alumina or silicic acid-alumina. Best results were achieved with the combination of nickel/tungsten-silicic acid/alumina. As demonstrated by the examples in the aforementioned reference, the hydrogenation is conducted as a discontinuous process in an autoclave with the use of long reaction periods. The weight ratio of the amount of raw material to the amount of catalyst employed ranges between 0.33 and 4; the reaction times range between 8 hours and 75 hours. Consequently, very low space-time yields are obtained for this method which are economically unacceptable for a technical, continuous process. Thus, due to the low activity of the catalysts employed, the process of DOS No. 2,164,951 is relatively very expensive.